1. Field of the Invention
The present invention relates to an optical head device for reading information from a multi-layered optical recording medium including a plurality of recording layers and a single servo layer. The invention also relates to a reading system for a multi-layered optical recording medium including the optical head device.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2007-4879 discloses focal point control, such as focus control or tracking control, for light beam irradiation of a multi-layered optical recording medium that includes a plurality of recording layers for recording information using holography.
In the invention disclosed in Japanese Patent Application Laid-Open No. 2007-4879, to record and read information on and from the multi-layered optical recording medium, a laser light source for a servo layer and a laser light source for the recording layers are separately provided. The light beams from these light sources are projected to the optical recording medium through a common objective lens to simultaneously perform recording and reading and servo control including focus servo and tracking servo.
The light beam for the servo layer is focused always on the same servo layer. However, the light beam for the recording layers must be focused on a predetermined one of the plurality of recording layers.
When a common objective lens is used for irradiation with light beams for servo control and recording and reading and for reception of the reflected light beams, as in the invention disclosed in Japanese Patent Application Laid-Open No. 2007-4879, the servo light beam and the recording and reading light beam must be separately focused at different timings. In addition, the information from the servo layer and the information from the recording layers must be separated from each other by modulating the laser light sources at different frequencies or turning the laser light sources on-off in a time sharing manner.
Therefore, when recording and reading operations are performed at different speeds such as 2× and 4× speeds, quick focusing is difficult, and the modulation frequency setting for the light sources is complicated. This is an obstacle to an increase in recording and reading speed. Moreover, since the servo light beam and the recording and reading light beam are simultaneously projected through the common objective lens and the reflected light beams are received also through the same, inter-layer stray light between the servo layer and the recording layer or between different recording layers may cause undesirable noise during information reproduction.